Reflecting mirror and optical pickup

ABSTRACT

A reflecting mirror with a reflective multi-layer coating capable reflecting light beams of discrete wavelengths, e.g., laser beams in the wave ranges of CD, DVD and mass storage optical discs, free of the problem of light absorption.

BACKGROUND OF THE INVENTION

1. Field of the Art

This invention relates to a reflecting mirror capable of reflectinglight beams of discrete wavelengths, and an optical pickup incorporatingthe reflecting mirror.

2. Prior Art

Generally, an optical pickup is constituted by a light source, polarizedbeam splitter, λ/4 wave plate, objective lens, APC (Auto Power Controland a photodetector. A light beam from a light source is fed to apolarized beam splitter to separate p-polarized light and s-polarizedlight. One of separated p- and s-polarized light beams is shed on anoptical disc while the other one is shed on an APC for detection ofsignals. The light beam to be shed on an optical disc is converted fromlinearly polarized light to circularly polarized light by a λ/4 waveplate and reflected off the optical disc, and further converted fromcircularly polarized light to linearly polarized light by the λ/4 waveplate before entering the polarized beam splitter again. Since thedirection of polarization is shifted 90 degrees at the time ofre-entering the polarized beam splitter, the incident light beam iseither transmitted through or reflected off and shed on a signaldetector. Generally, in an optical pickup of this sort, a light beam isprojected from a light source along a light path parallel with thesurface of an optical disc, and the parallel light path is turned towardthe optical disc by the use of a reflecting mirror (turning mirror).

Optical pickups employs a laser beam and thus a light source whichoutputs a laser beam. On the other hand, as the reflecting mirrormentioned above, there may be employed a reflecting mirror having ametal film deposited on a surface of a substrate or a reflecting mirrorhaving a dielectric multi-layer film coating formed by alternatelylaminating a high refractivity layer and a low refractivity layer oneafter another. Because a metal film is susceptible to corrosion,oxidation and bruises or other damages, and because of the ability ofefficiently reflecting off a specific wavelength, it has been thegeneral practice for an optical pickup to employ a reflecting mirrorwith a dielectric multi-layer coating.

In the case of a reflecting mirror with a dielectric coating, generallyTiO₂ and SiO₂ are used for the high and low refractivity layers,respectively. Namely, among various materials which are useful fordepositing the high refractivity layers, TiO₂ is used in most cases forits high refractivity. The large difference in refractivity between thehigh refractivity material TiO₂ and the low refractivity material SiO₂makes it possible to reduce the number of layers in producing adielectric coating which is capable of reflecting a laser beam of aspecific wavelength.

In this connection, recently optical pickups are required to cope withnot only CDs (Compact Discs) using a laser beam of 780 nm in wavelengthand DVDs (Digital Versatile Discs) using a laser beam of 650 nm but alsoto mass storage optical discs (using the so-called blue laser of 405 nmin writing and reading data). In a case where high and low refractivitylayers are formed of TiO₂ and SiO₂, with a large difference inrefractivity as mentioned above, a dielectric multi-layer coating withoptical characteristics of reflecting three discrete wavelengths can beformed by depositing a reduced number of high and low refractivitylayers. However, TiO₂ which absorbs light of short wavelength has aproblem that it invites degradations in efficiency of luminous energy.That is, when a light beam of 405 nm is cast on a dielectric multi-layercoating consisting of alternately laminated layers of TiO₂ and SiO₂,part of energy of incident light is absorbed by the action TiO₂ layers.Therefore, part of incident light is not reflected by the dielectricmulti-layer coating, resulting in a drop in efficiency of luminousenergy.

In this regard, Japanese Laid-Open Patent Application H3-12605 disclosesa reflecting mirror with a dielectric multi-layer coating which isarranged to avoid the above-mentioned problem of light absorption.

In the case of Japanese Laid-Open Patent Application H3-12605, thinlayers of a first group are formed on a substrate, and then thin layersof a second group are formed on the first group. The thin layers of thefirst group are formed by the use of a material which is absorptive oflight in an ultraviolet wavelength range, while the thin layers of thesecond group are formed by the use of a material which is not absorptiveof light in the ultraviolet wavelength range. Ultraviolet light raysincident on the reflecting mirror are reflected off by the thin layersof the second group, that is to say, incident ultraviolet light rays arereflected off without being absorbed by the mirror. On the other hand,as described in Japanese Laid-Open Patent Application H3-12605, the thinlayers of the first group are formed of a material which is absorptiveof light in an ultraviolet wavelength range. This is because the use ofa material which is absorptive of light in an ultraviolet wavelengthrange is effective for broadening a reflecting band width which appearsin an ultraviolet wavelength range.

In the case of a reflecting mirror with a dielectric multi-layer coatingwhich is formed by alternately laminating a high refractivity layer anda low refractivity layer one after another, lamination of a great numberof layers is necessary in order to impart optical characteristics forreflection of a plural number of discrete wavelengths. In JapaneseLaid-Open Patent Application H3-12605 mentioned above, the layers aredivided into a first group and a second group but as a whole the numberof the layers is not reduced.

SUMMARY OF THE INVENTION

In view of the foregoing situations, it is an object of the presentinvention to provide a reflecting mirror with a dielectric multi-layercoating which is free from the problem of light absorption and which isconstructed of a reduced number of layers.

According to the present invention, in order to achieve the above-stateobjective, there is provided a reflecting mirror virtually capable oftotally reflecting light beams of two or more discrete wavelengths,characterized in that the reflecting mirror comprises: a firstreflective multi-layer coating formed on a substrate plate byalternately depositing a first high refractivity layer absorptive oflight of short wave and a low refractivity layer;

a second reflective multi-layer coating formed over an incident side ofthe first reflective multi-layer coating, by alternately depositing abarely light absorptive second high refractivity layer having a lowerrefractivity than the first high refractivity layer and a lowrefractivity layer;

an incident light beam of short wave being reflected off by the secondreflective multi-layer coating while incident light beams of wave rangesother than short wave are reflected off by cooperative actions of thefirst and second reflective multi-layer coatings.

In an optical pickup according to the present invention, the abovereflecting mirror is incorporated in a light path to or from a lightsource, polarized beam splitter, λ/4 wave plate and photodetector.

The above and other objects, features and advantages of the presentinvention will become apparent from the following particulardescription, taken in conjunction with the accompanying drawings whichshow by way of example preferred embodiments of the invention. Needlessto say, the present invention should not be construed as being limitedto particular forms shown in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic illustration of construction of a reflectivemulti-layer coating of a reflecting mirror;

FIG. 2 is a graph showing reflectivity characteristics of an example inwhich ion plating is applied in forming a reflective multi-layer coatingcomposed of a first reflective multi-layer coating consisting ofalternately laminated TiO₂ and SiO₂ layers and a second reflectivemulti-layer coating consisting of alternately laminated Nb₂O₅ and SiO₂layers;

FIG. 3 is a graph showing reflectivity characteristics of an example inwhich ion plating is applied in forming a reflective multi-layer coatingcomposed of alternately laminated Nb₂O₅ and SiO₂ layers;

FIG. 4 is a graph showing reflectivity characteristics of an example inwhich vacuum deposition is applied in forming a reflective multi-layercoating composed of a first reflective multi-layer coating consisting ofalternately laminated TiO₂ and SiO₂ layers and a second reflectivemulti-layer coating consisting of alternately laminated Nb₂O₅ and SiO₂layers;

FIG. 5 is a graph showing reflectivity characteristics of an example inwhich vacuum deposition is applied in forming a reflective multi-layercoating composed of alternately laminated Nb₂O₅ and SiO₂ layers;

FIG. 6 is a schematic illustration showing an example of optical pickupapplying the reflecting mirror according to the present invention; and

FIG. 7 is a schematic illustration showing another example of opticalpickup.

DESCRIPTION OF PREFERRED EMBODIMENTS

Now, the present invention is described more particularly by way of itspreferred embodiments with reference to the accompanying drawings.Referring to FIG. 1, there is shown a reflecting mirror 1 with a firstreflective multi-layer coating 10 which is deposited on a substrateplate 30 like a glass substrate plate and a second reflectivemulti-layer coating 20 which is deposited on the first reflectivemulti-layer coating. Light incident on the reflecting mirror 1 isfirstly shed on the second reflective multi-layer coating 20. That is,the second reflective multi-layer coating 20 is formed on the side ofincident light, while the first reflective multi-layer coating 10 isformed on the side of the substrate plate 30. Both of the first andsecond reflective multi-layer coatings 20 and 30 are a dielectricmulti-layer coating formed by alternately depositing a high reflectivitylayer and a low reflectivity layer. At the same time, the first andsecond reflective multi-layer coatings 10 and 20 are reflective of lightof different wavelengths. Therefore, the first and second reflectivemulti-layer coatings 10 and 20 are each in the form of a dielectricmulti-layer coating which is formed by alternately depositing a highrefractivity layer and a low refractivity layer, but totally or partlydiffer from each other in constituent material and thickness of thealternately laminated layers. In this instance, the same material isused for low refractivity layers of the first and second reflectivemulti-layer coatings 10 and 20. On the other hand, high refractivitylayers of the first and second reflective multi-layer coatings 10 and 20are formed of different materials. Of course, different materials can beused in depositing low refractivity layers of the first and secondmulti-layer coatings 10 and 20.

As shown in FIG. 1, in the first place the first reflective multi-layercoating 10 deposited on top of the substrate plate 30, and then thesecond reflective multi-layer coating 20 is deposited on top of thefirst reflective multi-layer coating 10. As mentioned hereinbefore,first high refractivity layers 10H in the first reflective multi-layercoating 10 are formed of a different material from second highrefractivity layers 20H in the second reflective multi-layer coating 20.However, low refractivity layers L in the first and second reflectivemulti-layer coatings 10 and 20 are formed of the same or commonmaterial. In this instance, the first reflective multi-layer coating 10is imparted with optical characteristics of reflecting light in thevicinity of a DVD wave range (650 nm) as well as light in the vicinityof a CD wave range (780 nm) almost 100%. On the other hand, the secondreflective multi-layer coating 20 is imparted with opticalcharacteristics of reflecting light in the vicinity of a wave range ofmass storage optical discs (using blue laser light of 405 nm) almost100%.

The second reflective multi-layer coating 20 which is adapted to reflectlight of mass storage optical discs also has optical characteristics ofreflecting light in CD and DVD wave ranges to some extent. As seen inFIG. 1, the second reflective multi-layer coating 20 is formed on thelight incident side anterior to the first reflective multi-layer coating10. Therefore, light in CD and DVD wave ranges is shed firstly on thesecond reflective multi-layer coating 20 and thereby reflected off tosome extent. That is to say, light in CD and DVD wave ranges isreflected by cooperative actions of the first and second multi-layercoatings 10 and 20, and 100% reflection by the first reflectivemulti-layer coating 10 alone is not necessary.

High refractivity layers (second high refractivity layers 20H) and lowrefractivity layers (low refractivity layers L) of the second reflectivemulti-layer coating 20, which reflects off short waves in the vicinityof 405 nm, are formed of materials which are barely absorptive ofincident short wave light. It is known that light absorption takes placewhen short wave light is shed on a highly refractive dielectric coating.Shorter the wavelength, higher becomes the light energy. Therefore,short wave light with strong energy is shed on a high refractivitylayer, part of the energy is absorbed by the high refractivity layer, aphenomenon of “light absorption.” In order to suppress this phenomenon,the second high refractivity layers 20H in the second reflectivemulti-layer coating 20 are formed of a material which is barelyabsorptive of short waves.

Thus, light in a wave range of mass storage optical discs is reflectedoff by the second reflective multi-layer coating 20 without beingabsorbed by the coating, and light in CD and DVD wave ranges isreflected off by the first and second reflective multi-layer coatings 10and 20. In this regard, it is conceivable to apply the high refractivitylayers of the second reflective multi-layer coating 20 to the entirereflective multi-layer coating. However, the high refractivity layers20H of the second reflective multi-layer coating 20, which are barelyabsorptive of light, is lower in refractivity than the first highrefractivity layers 10H which are light-absorptive. Therefore, in casethe entire reflective multi-layer is constructed in the fashion of thesecond reflective multi-layer coating 20, a greater number of layers arerequired to realize a reflective multi-layer coating utilizing adifference in refractivity. The number of layers as a whole can bereduced in case the first and second reflective multi-layer coatings 10and 20 are deposited cooperatively as described above.

Light in a wave range of mass storage optical discs may not becompletely reflected by the second reflective multi-layer coating 20.Namely, it becomes necessary to deposit a greater number of layers inorder to get strictly 100% reflection of light in a wave range of massstorage optical discs by the second reflective multi-layer coating.Therefore, the second reflective multi-layer coating 20 may be tailoredto have a reflectivity akin to 100%, if not 100%. In this case, light ina wave range of mass storage optical discs is transmitted through thesecond reflective multi-layer coating 20, and shed on the firstreflective multi-layer coating 10 which is absorptive of light. However,the rate of light absorption by the first reflective multi-layer coating10 is not extremely high, so that a transmitted fraction (severalpercent) of light in the wave range of mass storage optical discs isabsorbed not entirely but partially in an extremely small amount. Thus,absorption of light in the wave range of mass storage optical discstakes place only at an ignorable rate, so that the second reflectivemulti-layer coating 20 is not necessarily required to have perfect 100%reflectivity with respect to light in the wave range of mass storageoptical discs. In the foregoing description, the reflecting mirror 1 hasbeen described in connection with light of three different wavelengthsor in the wave ranges of CD, DVD and mass storage optical discs.However, it is to be understood that application of the reflectingmirror 1 is not limited to wavelengths of these optical discs, andsimilarly applicable to reflection of two or more wavelengths involvinga wavelength which is susceptible to light absorption.

Thus, the reflecting mirror 1 according to the present invention employsa dielectric multi-layer coating which is constructed of a reducednumber of layers and which is so arranged as to eliminate the problem oflight absorption. Given below are Examples applying the above-describedembodiment of the invention.

EXAMPLE 1

In the above-described embodiment, a barely light absorptive material isselected for the low refractivity layers L which are commonly used inthe first and second reflective multi-layer coatings 10 and 20. In thiscase, a material of SiO₂ (with a refractivity of approximately 1.47)base is used. In this regard, the low refractivity layers L may beformed of SiO₂ alone or an SiO₂-base material which contains, forexample, a small amount (e.g., 5%) of Al₂ 0 ₃ or the like in addition tothe major component SiO₂, provided that an additional substance isbarely light absorptive. The first high refractivity layers 10H areformed by the use of TiO₂ which is light absorptive but has a highrefractivity (with a refractivity of approximately 2.45). The secondhigh refractivity layers 20H are formed by the use of Nb₂O₅ (with arefractivity of approximately 2.30) which is barely light absorptive.Although Nb₂O₅ is used for the second high refractivity layers 20H inthis Example, there may be employed other barely light absorptivematerials, for example, such as Ta₂O₃ (with a refractivity ofapproximately 2.1), ZrO₂ (with a refractivity of approximately 2.05),CeO₂ (with a refractivity of approximately 2.3), a lanthanum-titaniummixture oxide (Substance H4, a product of Merck), a zirconium-titaniummixture oxide (OH-5, a product of Optron), and the like. Consideringhigh refractivity, low light absorption and high weatherability, Nb₂O₅is preferred to be applied to the second high refractivity layers 20H.However, since the second reflective multi-layer coating 20 is impartedwith reflection characteristics through utilization of a difference inrefractivity between high and low refractivity layers, a material forthe second high refractivity layers 20H should be higher in refractivitythan SiO₂, a material of the low refractivity layers L.

Plotted in the graph of FIG. 2 are p- and s-polarized light reflectioncharacteristics of a reflecting mirror having a first reflectivemulti-layer coating consisting of 24 alternately deposited TiO₂ and SiO₂layers and a second reflective multi-layer coating consisting of 8alternately deposited Nb₂O₅ and SiO₂ layers. In this instance, the angleof incidence of input light is 45 degrees, and above-mentionedsubstances are deposited by ion plating. As seen in FIG. 2, almost 100%p- and s-polarization reflectivity is attained in each one of the threewave ranges for CD, DVD and mass storage optical discs (i.e., in thewave ranges of 405 nm, 650 nm and 780 nm).

In this instance, since Nb₂O₅ is used for the second high refractivitylayers 20H of the second reflective multi-layer coating 20, thereflecting mirror is free from the problem of degradations in efficiencyof luminous energy as caused by light absorption. Besides, light beamsin the CD and DVD wave ranges are reflected not solely by the firstreflective multi-layer coating 10 but by cooperative actions of thefirst and second reflective multi-layer coatings 10 and 20, which can bereduced in number of layers.

Next, in a comparative example, solely Nb₂O₅ is used for the highrefractivity layers while solely SiO₂ is used for the low refractivitylayers. Namely, in this comparative example, a reflective multi-layercoating is formed by alternately depositing Nb₂O₅ and SiO₂, withoutusing TiO₂. The Nb₂O₅ and SiO₂ layers were deposited on a substrateplate by ion plating, and the angle of incidence of input light is 45degrees. Plotted in the graph of FIG. 3 are p- and s-polarizationreflection characteristics of a dielectric multi-layer coatingconsisting of 46 alternately laminated Nb₂O₅ and SiO₂ layers. Similarlyto the case of FIG. 2, almost 100% reflectivity is attained in the threewave ranges.

As mentioned hereinbefore, Nb₂O₅ (with a refractivity of approximately2.3) is lower in refractivity as compared with TiO₂ (with a refractivityof approximately 2.45). Therefore, in case solely Nb₂O₅ is used for thehigh refractivity layers, the reflecting mirror is free from the problemof light absorption but needs a greater number of layers since thereflection characteristics are imparted through utilization of adifference in reflectivity between high and low refractivity layers.Therefore, in total 46 alternately laminated layers of Nb₂O₅ and SiO₂are required to impart the reflection characteristics shown in FIG. 3.In other words, although almost the same reflection characteristics areobtained in FIGS. 2 and 3, it suffices to deposit in total 32 layers onthe reflecting mirror 1 in case the dielectric coating is constructed ofthe first and second reflective multi-layer coatings but it becomesnecessary to deposit 46 layers in total in case the dielectric coatingis constructed of alternately deposited Nb₂O₅ and SiO₂ layers. Thus, ascompared with the comparative example, the combination of the first andsecond multi-layer coatings 10 and 20 can produce higher reflectioncharacteristics by deposition of a reduced number of high and lowrefractivity layers.

EXAMPLE 2

Instead of ion plating in Example 1, TiO₂, Nb₂O₅ and SiO₂ are depositedby vacuum deposition in this Example 2. As shown in FIG. 4 are p- ands-polarization reflection characteristics in a case where the firstreflective multi-layer coating 10 is formed by alternately depositingTiO₂ and SiO₂ in 24 layers in total and the second reflectivemulti-layer coating 20 is formed by alternately depositing Nb₂O₅ andSiO₂ in 10 layers in total. The angle of incidence of input light is 45degree, and all of above-mentioned substances are deposited by vacuumdeposition. As seen in FIG. 4, almost 100% reflection is attained ineach of the three wave ranges, i.e., in the wave ranges of CD, DVD andmass storage optical discs.

Shown in FIG. 5 are p- and s-polarization reflection characteristics ina case where vacuum deposition is applied in forming a reflectivemulti-layer coating by alternately depositing Nb₂O₅ and SiO₂ in 62layers in total. It is necessary to deposit 62 layers in total in orderto attain approximately 100% reflection of p- and s-polarizations ineach one of the three wave ranges as shown in FIG. 5. The reflectivemulti-layer coating which is constructed of alternately deposited Nb₂O₅and SiO₂ layers is free from the problem of light absorption, butrequires a greater number of layers due to a smaller difference inrefractivity as mentioned hereinbefore. A coating deposited by vacuumdeposition is lower in density as compared with a coating by ionplating, and therefore lower in refractivity. This is the reason whydeposition of 62 layers is required to obtain the reflectioncharacteristics as shown in FIG. 5.

On the other hand, in case the first and second reflective multi-layercoatings 10 and 20 are deposited to reflect light in a co-operative way,it is possible to eliminate the problem of light absorption and toattain the reflection characteristics as shown in FIG. 4 by depositionof a reduced number of layers (34 layers in total) even if vacuumdeposition is applied.

A minimal layer construction for the first and second multi-layercoatings 10 and 20 is shown in this example and in foregoing Example 1(32 layers in total in case of ion plating and 34 layers in total incase of vacuum deposition). However, smaller the number of layers,narrower becomes the band width of 100% reflection even if almost 100%reflection is attained in each one of the above-mentioned wave ranges.Therefore, a slight deterioration in reflectivity may occur whenincident light has angle dependency or under varying temperatureconditions. In this regard, the number of depositing layers may beincreased to some extent for the purpose of broadening the reflectionband width. However, it is desirable to control the number of depositinglayers, giving considerations to this question (the number of layers) inrelation with a desired reflection band width. Although vacuumdeposition an ion plating are applied in this Example 2 band in Example1, a sputtering or ion assist process may be applied if desired.

EXAMPLE 3

In this example, the above-described reflecting mirror 1 is applied toan optical pickup for CD (780 nm), DVD (605 nm) and mass storage opticaldiscs (405 nm). As shown in FIG. 6, the optical pickup is constituted bya light source 101, polarized beam splitter 102, λ/4 wave plate 103,reflecting mirror 104, objective lens 105 and photodetector 106.

The light source 101 emits selectively a laser beam of one of threewavelengths, i.e., a laser beam for CD, a laser beam for DVD or a laserbeam for mass storage optical disc. The laser beam from the light source101 firstly enters the polarized beam splitter 102 which splitspolarized components of incident light by transmission and reflectiondepending upon direction of polarization. In this instance, the laserlight emitted from the light source 101 is p-polarized light, and thepolarized beam splitter 102 is adapted to transmit p-polarized light andreflect off s-polarized light.(alternatively may be adapted to reflectoff p-polarization and to transmit s-polarization if desired). Thus, thelaser beam of p-polarization from the light source 101 is transmittedthrough the polarized beam splitter 102, and fed to λ/4 wave plate 103which converts linear polarization to circular polarization. Circularlypolarized light coming out of λ/4 wave plate 103 is reflected by thereflecting mirror 104 and converged to a predetermined position on anoptical disc D by the objective lens 105. Light reflected off theoptical disc D is fed again to λ/4 wave plate 103 via the objective lens105 and reflecting mirror 104. This time, circular polarization isconverted to linear polarization by λ/4 wave plate 103. Thus, on the wayback, original p-polarized light is converted to s-polarized light. Thatis to say, the laser beam of s-polarization is reflected off by thepolarized beam splitter 102 toward the photodetector 106 where incidentlight is converted to electric signals by photoelectric conversion.

In this instance, it is important for the reflecting mirror 104 to havealmost 100% reflection characteristics for each one of laser beams ofthree different wavelengths which are selectively emitted by the lightsource 101. Namely, the reflecting mirror 104 should have highreflectivity at all of operating wavelengths because all parts of thesystem are unchanged except that laser beams of different wavelengthsare emitted from the light source 101. Accordingly, the reflectingmirror 1 described in the foregoing embodiment is applied in thisExample as the reflecting mirror 104. As described above, since thereflecting mirror 1 is free of the problem of light absorption, light of405 nm can be totally reflected without being absorbed by the reflectingmirror 104. Besides, the reflecting mirror has almost 100% reflectivityat all of three operating wavelengths, so that it is suitable forapplication to optical pickups with a light source which selectivelyemits laser beams of three different wavelengths like the light source101.

EXAMPLE 4

Example 3 above employs the light source 101 which is adapted toselectively output laser beams of three different wavelengths, thisexample employs two light sources 201 and 202 as shown in FIG. 7 foremission of laser beams of three different wavelengths. Those componentssuch as λ/4 wave plate 103, objective lens 105 and photodetector 106 areidentical with the counterparts in Example 3. In the case of FIG. 7, thelight source 201 is a light source which selectively emits laser beamsof CD and DVD wavelengths, while the light source 202 emits exclusivelya laser beam of a wavelength for mass storage optical discs. If desired,the light source 202 may be used as a light source which selectivelyemits laser beams of two different wavelength, and the light source 201as a light source which emits exclusively a laser beam of onepredetermined wavelength. The polarized beam splitter 203 has opticalcharacteristics to transmit p-polarization in CD and DVD wave ranges andto transmit p-polarization in the wave range of mass storage opticaldiscs. On the other hand, the polarized beam splitter 204 has opticalcharacteristics to transmit p- and s-polarizations in CD and DVD waveranges, while transmitting p-polarization and reflecting s-polarizationin the wave range of mass storage optical discs. Laser beams which areemitted from the light sources 201 and 202 are all s-polarizations.Accordingly, an s-polarization laser beam of CD or DVD wave range, whichis emitted from the light source 201, is reflected toward an opticaldisc D by the polarized beam splitter 203. On the other hand, a laserbeam of a wave range of mass storage optical discs, which is emittedfrom the light source 202, is reflected toward a disc D by the polarizedbeam splitter 204. Although the polarized beam splitters 203 and 204have been described as having optical characteristics to transmitp-polarization and to reflect off s-polarization, they may be adapted toreflect p-polarization and transmit s-polarization if desired.

Thus, a light path for each one of the three kinds of laser beams forCD, DVD and mass storage optical disc is synthesized toward the λ/4 waveplate 103. Therefore, on the way to and on the way away from the opticaldisc D, each one of the three laser beams is reflected by the reflectingmirror 206, an application of the reflecting mirror 1 of theabove-described embodiment which can reflect each one of three laserbeams almost 100% free of the problem of light absorption.

1. A reflecting mirror virtually capable of total reflection of lightbeams of two or more discrete wavelengths, characterized in that saidreflecting mirror comprises: a first reflective multi-layer coatingformed on a substrate plate by alternately depositing a first highrefractivity layer absorptive of light of short wave and a lowrefractivity layer; a second reflective multi-layer coating formed overan incident side of said first reflective multi-layer coating, byalternately depositing a barely light absorptive second highrefractivity layer having a lower refractivity than said first highrefractivity layer and a low refractivity layer; an incident light beamof short wave being reflected off by said second reflective multi-layercoating while incident light beams other than short wave are reflectedoff by cooperative actions of said first and second reflectivemulti-layer coatings.
 2. A reflecting mirror as defined in claim 1,wherein a light beam of a wavelength in the vicinity of 780 nm or 650 nmis reflected off cooperatively by said first and second reflectivemulti-layer coatings, while a light beam of a wavelength in the vicinityof 405 nm is reflected off by said second reflective multi-layercoating.
 3. A reflecting mirror as defined in claim 1, wherein said lowrefractivity layer is formed by deposition of an SiO₂-base material,said first high refractivity layer is formed by deposition of TiO₂, andsaid second high refractivity layer is formed by deposition of at leastone of Ta₂O₅, Nb₂O₅, ZrO₂, CeO₂, a lanthanum-titanium mixture oxide anda zirconium-titanium mixture oxide.
 4. A reflecting mirror as defined inclaim 1, wherein said low refractivity layer is formed by deposition ofan SiO₂-base material, said first high refractivity layer is formed bydeposition of TiO₂, and said second high refractivity layer is formed bydeposition of Nb₂O₅; said second reflective multi-layer coating beingformed by alternately depositing Nb₂O₅ and SiO₂ in more than 8 layers intotal, and said first reflective multi-layer coating being formed byalternately depositing TiO₂ and SiO₂ in more than 24 layers in total. 5.A reflecting mirror as defined in claim 1, wherein said low refractivitylayer, first high refractivity layer and second high refractivity layerare deposited by an ion plating, vacuum deposition, ion assist orsputtering process.
 6. An optical pickup incorporating the reflectingmirror of claim 1 in a light path to or from a light source, polarizedbeam splitter, λ/4 wave plate and photodetector.